1. Field of the Invention
The present invention relates to a shift control system for an automatic transmission of a vehicle and more particularly, to an improved shift control system for a vehicle automatic transmission, which can prevent a shift shock of the automatic transmission.
2. Description of Related Art
Generally, a transmission increases or decreases rotating power and rotating speed of an engine according to a driving condition of a vehicle, and functions in an off-position between the engine and driving wheels for a certain period of time so as to warm-up the engine. The transmission industry classifies a manual transmission by manual operation, and an automatic transmission by automatic operation.
The automatic transmission related to the present invention is convenient to operate, enables the vehicle to start smoothly, to increase and decrease the car speed, and is less likely to stall when driven by a beginning driver. Therefore, this automatic transmission is a popular choice among drivers.
The automatic transmission comprises a "D" range which shifts in multiple stages according to a pushing degree of an accelerator pedal in driving of a vehicle, a "2" range which matches with a long downward slope of a driving road, and an "L" range which matches with an extreme downward slope and a long extreme curve.
The automatic transmission further comprises an "N" range which ignites the engine free of all elements of the engine, a "P" range which fixes an outputting shaft while the vehicle is parking, and an "R" range which operates a reverse motion of the vehicle.
The shift motion in the "D" range starts the vehicle in a 1-speed or 2-speed state without any relation of an opening degree of a throttle valve, and causes an up-shift to the 2-speed or 3-speed according to the car speed.
Also, if the throttle valve is gradually opened, a driving gear of the 1-speed, 2-speed, and 3-speed has an increasingly higher shift point thereof. If the car speed is down, the downshift automatically occurs in the 3-speed, 2-speed, and 1-speed. In this 3-speed or 2-speed state of the "D" range, if the accelerator pedal is pushed completely in order to suddenly accelerate the car speed, the automatic transmission conducts a kick down state by down-shifting in the 3-speed to 2-speed or the 2-speed to 1-speed over the shifting point thereof.
The automatic transmission is provided with a hydraulic control system for shifting by automatically operating brake bands of start gear unit and clutches according to the car speed, the engine loading degree, and the shifting lever.
Each valve of the hydraulic control system is controlled in an open loop system by a transmission control unit TCU and a fluid load of a line pressure can be controlled from the manual valves. Therefore, the star gear shifting device located at a rear portion of a torque converter conducts a shift of the upshift or the downshift.
In the "D" range of the driving function, the 1-speed to 4-speed, or recently, the 1-speed to 5-speed is accomplished in multiple stages. In detail, a rear clutch and a one-way clutch operate in the 1-speed shift, the rear clutch and a kick down brake band operate in the 2-speed shift operation, a front clutch and the rear clutch operate in the 3-speed shift operation, and an end clutch and the kick down brake band operate in the 4-speed shift operation.
Each element of the star gear device in order to accomplish the shift operation conducts one of the shift control solenoid valves operated by a control signal which is permitted from the TCU according to the driving state of the engine and the car speed, operates one of the shift control valves of valve bodies, and a pressure control solenoid valve controls the pressure which is permitted from each element of the vehicle.
However, such conventional shift control systems suffer from a number of problems, such as for example, a shift shock occurs since such conventional shift control systems do not utilize a feedback control method by the TCU due to an incomplete return operation of a kick down piston in the 3 to 2-speed downshift operation while a downshift proceeds to shift from 4-speed to 1-speed when the engine and the vehicle drive in a lower speed.